1. Field of the Invention
The present invention relates to a method of driving a solid state imaging device, and particularly to a method of driving an aspect ratio variable solid state imaging device. Specifically, the aspect ratio variable solid state imaging device includes an image area having light receiving elements arranged in a matrix in accordance with a specified aspect ratio (for example, 16:9) and vertical registers for vertically transferring signal charges in respective vertical lines of the light receiving elements, and at least one horizontal register for horizontally transferring signal charges transferred from respective vertical registers. In this imaging device, a signal according to the above aspect ratio (for example 16:9) is outputted in a usual manner while a signal according to another aspect ratio (for example, 4:3) smaller than the above aspect ratio can be also outputted by discharging signal charges in right and left unnecessary portions in the image area in a horizontal blanking period.
2. Description of the Related Art
The television broadcasting is mainly carried out at an aspect ratio of 4:3 at present. However, high definition broadcasting at an aspect ratio of 16:9 is also carried out, and a television receiver for receiving high definition broadcasting tends to be popularized. In addition, a television receiver called a wide vision which reproduces high definition broadcasting at an image quality slightly lower than the original high image quality of the high definition broadcasting is also extensively used because of its low cost. In other words, television receivers different in aspect ratio are present in the market and homes. As a result, a video camera and the like have been required to be matched with the above two aspect ratios.
To meet such a requirement, there has been known a technique applied to a CCD solid state imaging device used for a video camera as imaging means, wherein light receiving elements and the like are arranged in an image area in accordance with an aspect ratio of 16:9. When a signal is outputted at the 16:9 mode, the imaging device is read out in a usual manner, that is, signal charges from effective pixels in the image area are all read out. On the other hand, when a signal is outputted at an aspect ratio of 4:3, signal charges in right and left unnecessary portions in the image area (each side portion having a width of being about one-eighth of the total width of the image area) are discarded by the function of the solid state imaging device.
Such a prior art aspect ratio variable CCD solid state imaging device is shown in FIGS. 6A and 6B. FIG. 6A is a plan view showing the schematic configuration of the solid state imaging device, and FIG. 6B is a time chart showing a horizontal blanking pulse, and a horizontal drive pulse for driving a horizontal register.
In these figures, reference numeral 1 designates an image area in which light receiving elements 2, . . . , 2 constituting pixels are arranged in a matrix. Reference numeral 3 designates a vertical register provided in the image area 1 for each vertical line of the light receiving elements, which is adapted to vertically transfer signal charges from the light receiving elements in each vertical line.
The light receiving elements 2, . . . , 2 and the vertical registers 3, . . . , 3 are arranged in the image area 1 in accordance with an aspect ratio of 16:9. In this example, specifically, the number of effective pixels in the horizontal direction is 948, while the number of effective pixels in the vertical direction is 486. The effective pixels in the number of 948xc3x97486 are all to be reproduced when the solid state imaging device is used at the aspect ratio of 16:9. On the other hand, when the solid state imaging device is used at the aspect ratio of 4:3, signal charges in a left side unnecessary portion 1L and a right side unnecessary portions 1R in the image area are discharged, that is, they are not outputted as signals for the solid state imaging device. The signal charges from a middle portion (necessary portion) 1C are not discharged even when the solid state imaging device is used at the aspect ratio of 4:3.
It is to be noted that in FIG. 6A, the light receiving elements 2, . . . , 2 and the vertical registers 3, . . . , 3 are shown to be present only in the middle portion 1C; however, they are actually arranged at the same pitches in the left side unnecessary portion 1L and the right side unnecessary portion 1R. Each of the left side unnecessary portion 1L and the right side unnecessary portion 1R has a width being about one-eighth of the total width of the image area 1; while the middle portion 1C has a width being about three-fourth of the total width of the image area 1.
Reference numeral 4 designates a horizontal register for transferring signal charges vertically transferred from the vertical registers 3, . . . , 3 in the horizontal direction. The horizontal register 4 may be provided either on the upper side or on the lower side of the image area 1, and in this embodiment, it is provided on the lower side of the image area 1. Reference numeral 5 designates an output section provided on the output end side of the horizontal register 4 for converting signal charges into electrical signals (voltages). The output section 5 also serves as means for discarding unnecessary signal charges outputted from the horizontal register 4.
Next, the operation of the CCD solid state imaging device will be described with reference to FIG. 6B.
In the mode of the aspect ratio of 16:9, horizontal transfer is carried out by the horizontal register 4 during the horizontal scanning period in the same manner as in the usual CCD solid state imaging device. At this time, the frequency of a horizontal drive pulse for driving the horizontal register 4, that is, a horizontal drive frequency is, for example, 18 MHz. Then, during a horizontal blanking period, vertical transfer, that is, transfer of signal charges by the vertical registers 3, . . . , 3 in the vertical direction is carried out. By such one vertical transfer, signal charges in the vertical registers 3, . . . , 3 on one horizontal line are transferred into the horizontal register 4.
In the mode of the aspect ratio of 4:3, the operation is complicated more than the 16:9 mode. During the horizontal scanning period, signal charges in the middle portion 1C of the image area 1, that is, only the necessary signal charges are horizontally transferred and outputted. Then, during the horizontal blanking period, signal charges in the right side unnecessary portion 1R are discharged, followed by vertical transfer, and signal charges in the left side unnecessary portion 1L are discharged. Specifically, signal charges in the necessary portion 1C for the n-th horizontal line are first transferred. Subsequently, during the horizontal blanking period, signal charges in the right side unnecessary portion 1R for the n-th horizontal line are discharged, followed by vertical transfer for transferring signal charges for the (n+1)-th horizontal line into the horizontal register 4, and signal charges in the left side unnecessary portion 1L for the (n+1)-th line are discharged.
In summary, during the horizontal blanking period, drive of the horizontal register, vertical transfer, and drive of the horizontal register must be carried out.
After the above horizontal blanking period is ended, that is, in the subsequent horizontal scanning period, signal charges in the necessary portion 1C for the (n+1)-th line are horizontally transferred. It is to be noted that in the 4:3 mode, the frequency of the horizonal drive pulse of the horizontal register 4 in the horizontal scanning period, that is, the horizontal drive frequency is 13.5 MHz, and the frequency of the horizontal drive pulse for discharging signal charges in the horizontal blanking period is, for example, 54 MHz.
The above-described prior art aspect ratio variable CCD solid state imaging device has a disadvantage that in the 4:3 mode, the horizontal register 4 must be driven at a high speed for discharging signal charges in the unnecessary portion. Specifically, the horizontal drive frequency is only 18 MHz for the CCD solid state imaging device specialized for the 16:9 mode, and only 13.4 MHz for the CCD solid state imaging device specialized for the 4:3 mode. In the case of the aspect ratio variable CCD solid state imaging device, however, the horizontal register 4, when used in the 4:3 mode, must be driven at a very high speed, for example, at 54 MHz for discarding signal charges in the unnecessary portion during the horizontal blanking period. This presents inconveniences in that unnecessary radiation is increased; transfer speed of the horizontal register is made higher; power consumption of the horizontal register is enlarged; and a fraction defective in transfer of the horizontal register is increased.
More specifically, the higher the frequency of the drive pulse, the more the unnecessary radiation, and the increased unnecessary radiation exerts adverse effect on the interior and exterior of the CCD solid state imaging device and it also increases power consumption of the horizontal register. As a result, it is required to prevent the drive frequency from being increased or to reduce a ratio of a term in which the drive frequency is high relative to one horizontal period.
In general, the increased transfer speed tends to cause a failure in transfer of the horizontal register, and accordingly, to prevent the generation of a failure in the horizontal register even at a high transfer speed, a very high performance is required for the horizontal register. The horizontal register capable of satisfying such a high performance is difficult to be obtained. For these reasons, it is required to lower the horizontal drive frequency of the horizontal register for avoiding such inconveniences.
In actually, such a technique as to meet the above requirement has been not known.
A primary object of the present invention is to provide a method of driving a solid state imaging device capable of solving the above-described problems in the prior art. Specifically, the solid state imaging device of the present invention includes an image area having light receiving elements arranged in a matrix in accordance with a specified aspect ratio and vertical registers for vertically transferring signal charges in respective vertical lines of the light receiving elements, and at least one horizontal register for horizontally transferring signal charges transferred from respective vertical registers. In this imaging device, a signal according to the above aspect ratio is outputted in a usual manner while a signal according to another aspect ratio smaller than the above aspect ratio can be also outputted by discharging signal charges in right and left unnecessary portions in the image area in a horizontal blanking period.
A specific object of the present invention is to provide a method of driving the above solid state imaging device, which is capable of lowering the frequency of a drive pulse of the horizontal register for discharging signal charges in the unnecessary portion at a small aspect ratio or shortening a term of discharging signal charges in the unnecessary portion in each horizontal period.
Another specific object of the present invention is to provide a method of driving the above solid state imaging device, which is capable of detecting a black level for each horizontal period.
To achieve the above objects, according to a preferred mode of the present invention, there is provided a method of driving a solid state imaging device, wherein a signal at a small aspect ratio is outputted by the steps of horizontally transferring signal charges in a necessary portion for the n-th line; horizontally transferring signal charges in an unnecessary portion on the side far from the output end of a horizontal register for the n-th line up to an output end section of the horizontal register; in a horizontal blanking period, transferring signal charges for the (n+1)-th line into the horizontal register by vertical transfer through vertical registers, thereby mixing the signal charges in the unnecessary portion on the side far from the output end of the horizontal register for the n-th line with signal charges in an unnecessary portion on the side near the output end of the horizontal register for the (n+1)-th line; discharging the mixed signal charges in the unnecessary portions from the horizontal register during the horizontal blanking period; and transferring and outputting signal charges in the necessary portion for the (n+1)-th line.
In this method, at the small aspect ratio, signal charges in an unnecessary portion on the side far from the output end of a horizontal register for the n-th line is mixed with signal charges in an unnecessary portion on the side near the output end of the horizontal register on the (n+1)-th line, and then the mixed signal charges are discharged. Accordingly, there can be eliminated horizontal transfer for discharging signal charges in an unnecessary portion directly after starting each horizontal blanking period, that is, horizontal transfer in the front side of the horizontal blanking period, which has been carried out in the prior art method of driving an aspect ratio variable solid state imaging device. In the present invention, horizontal transfer for discharging signal charges in unnecessary portions may be carried out only at the rear side of the horizontal blanking period.
Accordingly, the horizontal drive frequency can be significantly reduced, for example, to be half, or a time required for transferring the horizontal register at a high speed for discharging signal charges in an unnecessary portion in each horizontal blanking period can be significantly reduced, for example, to be half.
With this configuration, it becomes possible to reduce unnecessary radiation; to lower the probability in generating a failure in transfer of the horizontal register; to eliminate the necessity of excessively increasing a high speed transfer performance of the horizontal register; and to shorten a generating time of unnecessary radiation in each horizontal period.
According to another preferred mode of the present invention, there is provided a method of driving a solid state imaging device, wherein a drain region is provided in the horizontal register on the side opposite to the image area. When a signal in accordance with a small aspect ratio is outputted, part or all of signal charges mixed at the output end section of the horizontal register are discharged in the drain region.
In this method, transfer for discharging signal charges in unnecessary portions by the horizontal register may be carried out only at the rear side of the horizontal blanking period, and further since at least part of signal charges in unnecessary portions are discharged into the drain region, the amount of signal charges in the unnecessary portions required to be transferred by the horizontal register can be controlled not to exceeds the allowable value of the horizontal register.
Specifically, in the case where the drain region is not provided, since in the present invention signal charges in the right side unnecessary portion on one line is mixed with signal charges on the left side unnecessary portion for the next line, the amount of the mixed signal charges stored in the horizontal register becomes larger, and consequently, the allowable value for processing signal charges in the horizontal register must be set to be larger for smoothly transferring the mixed signal charges. In this method, however, since all of the mixed signal charges or the portion thereof exceeding the allowable value are discharged into the drain region, the amount of signal charges required to be transferred in the horizontal register can be reduced. As a result, there can be eliminated the necessity of excessively increasing the allowable value for processing signal charges in the horizontal register.
According to a further preferred mode of the present invention, there is provided a method of driving a solid state imaging device, wherein a drain region is provided in the horizontal register on the side opposite to the image area. In this method, a signal in accordance with a small aspect ratio is outputted by the steps of transferring signal charges for the n-th line by vertical registers; discharging part or all of signal charges in an unnecessary portion on the side far from the output end of the horizontal register into the drain region; transferring signal charges for the (n+1)-th line into the horizontal register by vertical transfer for mixing in the horizontal register the remaining portion of the signal charges in the unnecessary portion on the side far from the output end of the horizontal register with signal charges in an unnecessary portion on the side near the output end of the horizontal register for the (n+1)-th line; and transferring and discharging the mixed signal charges.
In this method, the transfer by the horizontal register for discharging signal charges in unnecessary portions may be carried out only at the rear side of the horizontal blanking period, and since signal charges in the unnecessary portion on the side near the output end of the horizontal register are transferred to the output end section by the horizontal register and can be discharged into the drain region before vertical transfer, even when they are mixed with signal charges on the unnecessary portion on the side near the output end of the horizontal register for the next line, the amount of the mixed signal charges are not excessively increased, thus suppressing a trouble in that the signal charges overflow the vicinity of respective bits.
According to still a further preferred mode of the present invention, there is provided a method of driving a solid state imaging device, wherein an optical black area is provided so as to be adjacent to an output side in horizontal transfer in an image area, and a horizontal register includes a first horizontal register portion corresponding to the image area and a second horizontal portion corresponding to the optical black area, the second horizontal register portion being connected to the output end of the first horizontal register portion and being connected at its output end side an output stage including a charge detecting portion. In this method, a signal in accordance with a small aspect ratio is outputted by the steps of outputting signal charges in a necessary portion for the n-th horizontal line through the first and second horizontal register portions; transferring unnecessary signal charges on the side far from the output end of the horizontal register up to the output end of the second horizontal register portion; stopping the first horizontal register portion and driving the second horizontal register portion to discharge the unnecessary signal charges in the second horizontal register portion; transferring signal charges in the image area and the optical black area for the (n+1)-th horizontal line into the first and second horizontal register portions by the vertical registers for mixing the unnecessary signal charges remaining in the output end of the first horizontal register portion with unnecessary signal charges on the side near the output end of the horizontal register for the (n+1)-th horizontal line; driving the first and second horizontal register portions to output the signal charges in the optical black area; and discharging the mixed signal charges until ending of the horizontal retrace period and transferring the necessary signal charges such that the head thereof reaches the output end of the horizontal register.
According to this method, in the state that unnecessary signal charges on the side opposite to the output end of the horizontal register for the n-th horizontal line remain in the output end section of the second horizontal register portion, signal charges for the (n+1)-th horizontal line are transferred into the first horizontal register portion, so that signal charges in the optical black area are taken in and the above unnecessary signal charges are mixed with the unnecessary charges on the side of the output end of the horizontal register for the (n+1)-th horizontal line. After that, the first horizontal register portion is driven to take out the signal charges in the optical black area (signal charges representing a black level) and then the mixed signal charges are discharged.
As a result, the drive frequency of the horizontal register for discharging unnecessary signal charges can be reduced to be half, and further the black level in the horizontal period can be detected.